WASHINGTON — The National Security Agency acted on its own authority, without a formal directive from President Bush, to expand its domestic surveillance operations in the weeks after the Sept. 11 attacks, according to declassified documents released Tuesday.

The NSA operation prompted questions from a leading Democrat, Rep. Nancy Pelosi of California, who said in an Oct. 11, 2001, letter to a top intelligence official that she was concerned about the agency’s legal authority to expand its domestic operations, the documents showed.

Pelosi’s letter, declassified at her request, showed much earlier concerns among lawmakers about the agency’s domestic surveillance operations than had been previously known. Similar objections were expressed by Sen. Jay Rockefeller, D-W.Va., in a secret letter to Vice President Cheney nearly two years later.

The letter from Pelosi, the House minority leader, also suggested that the National Security Agency, whose mission is to eavesdrop on foreign communications, moved immediately after the Sept. 11 attacks to identify terror suspects at home by loosening restrictions on domestic eavesdropping.

The congresswoman wrote to Lt. Gen. Michael V. Hayden, then head of the NSA, to express her concerns after she and other members of the House and Senate Intelligence Committees received a classified briefing from Hayden on Oct. 1, 2001, about the agency’s operations.

Pelosi, then the ranking Democrat on the House Intelligence Committee, said, “I am concerned whether, and to what extent, the National Security Agency has received specific presidential authorization for the operations you are conducting.”

The answer, Hayden suggested in his response to Pelosi a week later, was that it had not.

“In my briefing,” he wrote, “I was attempting to emphasize that I used my authorities to adjust NSA’s collection and reporting.”

Bush administration officials said Tuesday that Hayden, now the country’s second-ranking intelligence official, had acted on the authority previously granted to the NSA, relying on a 1981 intelligence directive known as Executive Order 12333.

“He had authority under EO 12333 that had been given to him, and he briefed Congress on what he did under those authorities,” said Judith A. Emmel, a spokeswoman for the Office of the Director of National Intelligence. “Beyond that, we can’t get into details of what was done.”

In 2002, President Bush signed a new executive order specifically authorizing NSA to eavesdrop without warrants on the international communications of Americans inside the United States who the agency believed were connected to al-Qaida. The disclosure of the domestic spying program last month provoked an outcry in Washington, where congressional hearings are planned.

Hayden’s October 2001 briefing was one of the first glimpses into the expanded but largely hidden role that the NSA would assume in combating terrorism over the last four years.

During the briefing, Pelosi wrote to Hayden, “you indicated that you had been operating since the September 11 attacks with an expansive view of your authorities” with respect to electronic surveillance and intelligence-gathering operations.

“You seemed to be inviting expressions of concern from us, if there were any,” Pelosi wrote, but she said that the lack of specific information about the agency’s operations made her concerned about the legal rationale justifying it.

One step that the NSA took immediately after the Sept. 11 attacks, Pelosi wrote in her letter, was to begin forwarding information from foreign intelligence intercepts to the FBI for investigation without first receiving a specific request from the bureau for “identifying information.”

In the past, under so-called minimization procedures designed to guard Americans’ privacy, the NSA’s standard practice had been to require a written request from a government official who wanted to know the name of an American citizen or U.S. person who was mentioned or overheard in a wiretap.

Parts of the letters from Pelosi and Hayden concerning other specific aspects of the NSA’s domestic operation were blacked out because they remain classified. But officials familiar with the unredacted letters said they refer to other aspects of NSA’s domestic eavesdropping program.

In the weeks after the Sept. 11 attacks, the agency began monitoring telephone calls and email messages between the United States and Afghanistan to track possible terror suspects. That program led to the broader eavesdropping operation on other international communications, officials have said.

The NSA has also tapped into some of the United States’ main telecommunications arteries to trace and analyze large volumes of phone and email traffic to look for patterns of possible terrorist activity.

Marc Rotenberg, director of the Electronic Privacy Information Center, said the new documents, along with previous reports of objections to the program from Sen. Rockefeller and James B. Comey, the former deputy attorney general, underscore the need for a comprehensive investigation.

“There’s an increasing picture of concern, if not outright opposition, within the government,” Rotenberg said. “But we can’t second-guess anyone’s actions on a document-by-document basis,” particularly if the documents are released only in part, he added.

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The way in which the agency’s role has been expanded has prompted concern even from some of its former leaders, like Bobby R. Inman, a retired admiral who served as NSA director from 1977 to 1981. He said that while he supports the decision to step up eavesdropping against potential terrorists immediately after the 2001 attacks, he said the Bush administration should have tried to change the Foreign Intelligence Surveillance Act to provide explicit legal authorization for what NSA was doing.

“What I don’t understand is why when you’re proposing the Patriot Act, you don’t set up an oversight mechanism for this?” he said in an interview. ” I would have preferred an approach to try to gain legislation to try to operate with new technology and with an audit of how this technology was used.”

Inman called the uproar over the warrantless eavesdropping “sad, if not a tragedy, for the agency.” Though the NSA program operated under an executive order from Bush, he said, many Americans believe the agency is “somehow acting illegally to spy on Americans.”

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